Huge Idaho snowpacks a sign of big runoff to come

Image courtesy the Idaho Department of Water Resources.

With only a month to go before the water year officially begins on April 1, many south and central Idaho river basins stand at 150 to 200 percent of average snowpack, with more snow in the near-term forecast.

The Big Wood, Little Wood and Big Lost river basins in particular are far above average at 184 percent, 196 percent and 190 percent of average snowpacks, respectively. 

Nearly as impressive are the Boise basin (144 percent), Salmon basin (131 percent), Owyhee (140 percent), Henry's Fork (126 percent) and, notably, the upper Snake River basin above Palisades Reservoir (162 percent). The upper Snake River basin is a significant source of water for south Idaho farmers, and its large reservoirs at American Falls, Palisades and Jackson, among a few additional smaller storage pools, can accommodate 4 million acre feet of water. Runoff hasn't yet begun, but the system is already 70 percent full.

A similar situation is unfolding in the Boise River basin where three big reservoirs can hold about 1 million acre feet. Collectively, Arrowrock, Lucky Peak and Anderson Ranch reservoirs are 60 percent full.

“With water in Idaho it is often feast or famine, and this abundance of snow is great news for Idaho's farmers and whitewater boaters,” said IRU Executive Director Kevin Lewis. “It could also pose a challenge, however, for managers of our dams and irrigation infrastructure.”

Paddlers will also have to exercise extreme caution, Lewis said. It’s too early to know for sure, and will depend in part on how fast the weather warms up this spring, but flows could approach or exceed all-time records.

Early releases of up to 6,000 cfs have been ongoing on the Boise River for a couple weeks now, and a warm snap in February caused widespread flooding on low-elevation streams across southern Idaho.  This is likely just the beginning of a big runoff season that could pose challenges for municipalities, irrigation districts and dam managers throughout the region.

Farther north, the Clearwater, St. Joe and Coeur d'Alene river basins range from 89 to 97 percent of average. 

The National Weather Service is forecasting significant additional precipitation across all parts of Idaho in the coming 10 days.